1. Field of the Invention
This invention relates to a burn-resistant plenum cable.
2. Description of Related Art
Plenum cable is cable used for data and voice transmission that is installed in building plenums, i.e. the spaces above dropped ceilings or below raised floors that are used to return air to conditioning equipment. The cable comprises a core which performs the transmission function and a jacket over the core. Typical core constructions include a plurality of twisted pairs of insulated wires or coaxially-positioned insulated conductors.
Cable jackets of polyvinyl chloride (PVC) and flame retardant additives are known for plenum cable, but the resultant compositions do not pass the NFPA-255 burn test (Surface Burning Characteristics of Building Materials), that requires both non-flammability and low-to-no smoke. This burn test is more severe than the burn test UL-910 (NFPA-262). UL 2424, Appendix A, provides that cables tested in accordance with NFPA-255 must have a smoke developed index (hereinafter Smoke Index) of no greater than 50 and a flame spread index (Flame Spread Index) of no greater than 25.
Cable jackets of tetrafluoroethylene/hexafluoropropylene (FEP) copolymer are also known that do pass the NFPA-255 burn test. Such FEP has a melt flow rate (MFR) of 2-7 g/10 min, which means that it has a high melt viscosity. Because of this high melt viscosity, this FEP has the disadvantage of high production cost cable jacket, because this FEP is only capable of being extruded at a rate (line speed) of up to about 120 ft/min. Higher MFR (lower melt viscosity) FEP has been tried as cable jacket, but such jacket does not pass the NFPA-255 test. As the MFR increases above 10 g/10 min, the resultant lower melt viscosity of the FEP causes it to drip and smoke, resulting in a Smoke Index of greater than 50.